Chromosomes AP Biology Ms. Day

RECALL… DNA & RNA = nucleic acids Central Dogma of Biology DNA (has genes)  RNA  PROTEIN Proteins give you traits Gene = a section of DNA that codes for a protein or RNA

Gene Expression: 2 parts Involves DNA, RNA and the Central Dogma of Biology DNA  RNA  PROTEIN DNA  RNA called transcription Going from DNA nucleotides  RNA nucleotides RNA  protein called translation Going from RNA nucleotides  amino acids

THINK: Translation happens “LATER” GENE EXPRESSION

Why is Cell Division Important? Unicellular organisms Reproduce by cell division  increasing the population. 100 µm (a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM). Figure 12.2 A

Why Do Multicellular Organisms Depend on Cell Division? Development & Growth Repair (ex: tissue renewal) Maintenance 20 µm 200 µm (b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM). (c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

Cell Division (Mitosis) Makes 2 genetically identical 2 daughter cells from 1 parent cell Before cells divide Cells complete DNA replication ensures that each daughter cell receives an exact copy of the DNA

Organization of the Genetic Material All of a cell’s DNA (genetic information) is called its genome In prokaryotes Genome=single, long DNA molecule in a circle In eukaryotes Genome = several DNA molecules grouped in clumps (called chromosomes)

Forms of DNA DNA (in nucleus of eukaryotes) can be in 2 forms Chromatin : DNA is not tightly packed together (loosely coiled) Occurs during interphase (most of cell’s life) Chromosomes : tightly packed together (TIGHTLY coiled/condensed) Occurs during mitosis (cell division)

How Does DNA Fit into a Small Cell? Eukaryotic chromosomes stored within the cell's nucleus Prokaryotic chromosomes stored within the nucleoid region DNA is coiled around proteins called histones Group of 8 histones = nucleosome helps form a “supercoil”

Nucleosomes Like beads on a string

Tails have positive charge; DNA has negative charge Histone Tails + - Tails have positive charge; DNA has negative charge

Histone Modification Adding acetyl group to the tail (acetylation) Neutralizes charges Making DNA less tightly coiled Increasing transcription “opens” up DNA

DNA Modification Adding a methyl group to the DNA (methylation) Maintains the positive charge Makes DNA more coiled Reduces transcription

Gene is “active” Switched on Open chromatin Transcription can happen Unmethylated cytosines (white circles) Acetyl groups ARE on histones Gene is “inactive” No transcription Methylated cytosines (red circles) No acetyl groups on histones

Making Chromosomes **Occurs right before cell division (mitosis) Steps: DNA copies itself through DNA Replication The DNA coils (wraps) around proteins called histone proteins. Chromosomes are made.

Two shapes of chromosomes Occurs right before cell division (mitosis) Chromosome Structure Two shapes of chromosomes Single chromosome (1 copy of DNA); “V” shaped Double Chromosome (2 copies of DNA); “X” shaped; a.k.a-duplicated chromosome Occurs right before cell division (mitosis)

Chromosomes and Cell Division To prepare for cell division (mitosis) DNA is replicated

Separation of sister chromatids Each duplicated chromosome has two sister chromatids, which separate during cell division 0.5 µm Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Sister chromatids Centromeres A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule. Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. Figure 12.4

Double Chromosome Structure Kinetochore attaches to spindle fibers Sister

Another view…

Chromosomes Every eukaryotic species has a characteristic, unique # of chromosomes in EACH cell nucleus Ex: Humans have 46 chromosomes # of chromosomes does NOT necessarily equal complexity

Different Types of Cells There are 2 types of cells in the human body Somatic cells: all body cells 2. Gametes: reproductive cells (sperm and egg)

Different cell types  different cell divisions Eukaryotic cell division consists of 1. Mitosis Occurs in somatic (body) cells 2. meiosis Occurs in GERM LINE CELL (a special somatic cell in gonads) CREATES gamete cells

Chromosome Number Chromosomes come in PAIRS Gametes (sperm/egg) have 1 copy of each chromosome called haploid Somatic (body) have 2 copies of each chromosomes called diploid Chromosomes come in PAIRS All pairs of chromosomes differ in size, shapes, and set of genes.

Haploid vs. Diploid Haploid= when a cell has 1 copy of each chromosome. Expressed as n = 23 Diploid= when a cell has 2 copy of each chromosome. (a.k.a TOTAL # of C’s in organisms) Expressed as 2n = 46 n Haploid sperm cell Haploid egg cell n Fertilization 2n 1st diploid Zygote cell of an organism

Set of Chromosomes Each chromosome set contain 2 “homologues” or “twins” Chromosomes have a “twin” or matching pair Homologous Chromosome Pair = 2 chromosomes that are similar in shape, size, and genes. Each homologue in a pair comes from each parent!

SIMILAR: 1. Size 2. Shape 3. Genes they carry DIFFERENT specific gene info (order of ATCG’s) can differ

Sets of Chromosomes in Human Cells In humans Each somatic cell has 46 chromosomes Chromosomes come in pairs or sets One set of chromosomes comes from each parent Human somatic cells have 23 PAIRS of chromosomes All pairs of chromosomes differ in size, shapes, and set of genes.

Chromosomes Determine Your Gender Humans have 23 pairs of chromosomes 22 pairs are autosomes (chromosomes that are NOT involved in making gender) Also called homologous chromosome pairs 1 pair are sex chromosomes (NOT HOMOLOGOUS; different genes) The sex chromosomes are “X” and “Y” “XX” is a girl “XY” is a boy X Egg cells ALWAYS have “X” chromosome Y X Y

Chromosomal Analysis: A karyotype Pair of homologous chromosomes Centromere Sister chromatids an ordered, visual representation of ALL chromosomes in a human cell Arranged according to homologous pairs

Karyotypes (pictures of chromosomes) Used to study chromosomes & look for LARGE mutations (ex: chromosomal translocations, deletions, extra chromosomes) To make a karyotype 1. scientists take a picture of a cell’s chromosomes during metaphase and STAIN the chromosomes 2. Cut out the chromsomes and arranged them according to size and gene band pattern 3. They # the chromosome pairs **Use #1 through 23 4. #23 = ALWAYS sex chromosome pair (not homologous-NOT similar in genes it holds)